Wave filter



W. P. MASON slept. 2s, 1937.

WAVE FILTER Filed July 2, 1955 Patented Sept. 28, 1937 WAVE FILTER Warren P. Mason, West Orange, N. J., assignor. to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application July 2, 1935, Serial No. 29,463

7 Claims.

This invention relates to wave filters and more particularly to lattice type wave filters using electrically driven mechanical vibratory devices as impedance elements.

The principal objects of the invention are the reduction of the number of mechanical vibratory elements in a given filter structure and the simpliiication of the construction in the case of balanced types of circuit.

It is well-known that the lattice type wave filter possesses the advantage of complete generality in respect of the transmission characteristics it is able to provide. This advantage is accompanied, however, by the disadvantages of requiring a relatively large number of impedance elements and a high degree of balance between the impedances ofv certain of the branches. Where mechanical vibratory devices such as piezoelectric crystals or electromagnetically driven vibrators are included in the lter circuit to secure the advantages `arising' from their low energy dissipation, the cost of production and adjustment of these devices mairesI it desirable that their number f be reduced to a minimum. v

In accordance with the present invention, the number of vibratory elements required in a symmetrical lattice type filter is reduced by coupling a single element to each pair of similar branches in such manner that each vibratory element affects equally the impedance of the two branches to which it is coupled. 'I'he electromechanical coupling may be either electrostatic or electromagnetic, the former being suitablewhere piezoelectric crystals are used as the mechanical vibratory elements and the latterr being better adapted for the coupling of metallic resonators, for example, tuning forks or vibrating rods.

The nature of the invention will be more fully understood from the following detailed description and by reference to the attached drawing, of which- Fig. 1 shows schematically a wave filter in accordance with the invention using piezoelectric crystals as electromechanical impedances;

Fig. 2 shows a form of piezoelectric crystal vibrator in accordance with the invention;

Fig. 3 illustrates characteristics of the device of Fig. 2;

Fig. 4 shows a magnetostrictive vibrator in accordance with the invention;

Figs. 5 and 6 illustrate characteristics of the device of Fig. 4;`

Fig. 7 is a schematic arrangement of a lter in accordance with the invention using magnetostrictive vibrators; and

Figs. 8 and 9 illustrate characteristics of the filter of Fig. 7.

Referring to Fig. 1, the embodiment of the invention illustrated is a lattice type piezoelectric crystal filter comprising two piezoelectric crystals I0 and II which act as mechanical vibratory elements and which are so coupled to the electrical portion of the network that each crystal is included in two similarly disposed branches of the lattice. Thus, crystal I0 is included in each of the line branches and crystal II in each of the lattice branches. The input terminals of the filter are designated I and 2 and the output terminals 3 and 4.

The crystals may be rectangular plates cut with the plane of the rectangle perpendicular to the electric axis of the crystal and with the longer edge of the rectangle parallel to the mechanical axis of the crystal. Crystals cut in this fashion and provided with electrodes on the large faces of the rectangle vibrate longitudinally under electrical excitation and, when proportioned to produce resonances in the relatively low frequency range used in carrier telephony, have dimensions convenient for mechanical mounting. Other Well-known types of crystal cutmay also be used and under certain conditions these may be preferred. In the filter illustrated in Fig. 1 the crystals are of the rectangular type described above, but for convenience are shown in end elevation.

Crystal I0 is provided with two pairs of electrodes I2, I2', and I3, I3 on its opposite faces, electrodes I2 and I2 being directly opposite each other on the upper half of the crystal and electrodes I3 and I3 being similarly located on the lower half of the crystal. These electrodes may be of silver, plated directly on to the crystal, and may be applied by plating the two surfaces all over and afterwards removing a narrow strip of the plating along the center of each face. It is generally desirable also to remove narrow strips of the plating around the edges of the crystal. Crystal II is likewise provided with two corresponding pairs of electrodes I4, I4 and I5, I5. The electrodes I2 and I2' of crystal Ill are connected respectively to terminals I and 3 and electrodes I3 and I3' to terminals I and 2 respectively. Electrodes Il and It of crystal II are connected respectively to terminals I and l and electrodes I5 and I5 to terminals 3 and 2 respectively. The perspective drawing in Fig. 2 shows the form of crystal I0, the arrangement of the electrodes and their connections tothe tory structure their behavior in the filter circuit may be studied most conveniently by considering each as being eqJIv/alent to two crystals obtained by dividing the single crystal longitudinally along This would give two similar line branch crystals and the gap between the adjacent electrodes.

two similar lattice branch-crystals. If fourseparate crystals were used it would not-be necessary to pay any attention to the manner of interconnecting the electrodes and the filter terminals,` but when they are paired in accordance with the y f invention to form unitary structures it is neces- .included in the lines external to the lattice for this sary to ensure that the exciting voltages applied to each half will be so related that both halves I will vibrate simultaneously in the same direction'.

mode of each half. It w111 be observed that in" the case of crystal Ill the lower electrodes lli and- I3' are connected to the input and output ter*-y minals in reverse fashion to electrodes I2 and I2.'-

and that a similar reversal is present in theponf-f nections of the electrodes of crystal I I.

'I'he electrical equivalent of lcrystal I0 is illusg-v trated in Fig. 3. It comprises two similar impedances each consisting of a .simple resonantcir cuit shunted by a capacity, the one impedance being connected between terminals I and 3 and the other between terminals 2 and 4. 'I'he twof i dances in the `electrical e uivalent are free mpe q Y the polarizing magnet to permit free vibration.

from mutual coupling although in the actual device the same mechanical resonator functions in each path.

The values of the inductancesl and the capacities appearing in the electrical equivalent may be determined from the crystal dimensions by'rst determining the electrical equivalent of the whole crystal on the assumption that the two electrodes on each surface are connected together to form single electrodes covering substantially the whole surface. The impedance of each half of the crystal will then be twice that of the whole crystal. In Fig. 3 each impedance comprises a resonant branch includes an inductance 2L1 and a capacity C1/2, and a shunt capacity C11/2. In terms of the dimensions of the crystal these quantities have substantially the following values:

2L1== t Henries width, and thickness of the crystal measured in centimeters.

The requirements for combining two pairs of l nance of the vibrator.

the line branches coincides with the anti-resonance -of the lattice branches or vice versa. Equations (l) permit the crystal dimensions to vloe determined'to meet this condition.

In theaforementioned copending application -it'is al`s'o.1s hovvi 1l that the use of inductances in series with eachcrystal or, alternatively, in series withjeachline 'external to the lattice, permits wideribandsto be obtained without sacrifice of the sharpnessfofselectivity resulting from the. -lowxdissipationin the piezo-electric crystals. Fig.

1"shows..four V'equal series inductances of value purpose.

l Another typeofelectromechanical impedance. uitablefor-use in the lters of the invention is Il and I "I andprovided with exciting windings vI Bf-anl ;I 9- -'for 'thealternating currents. These v '.vviridirigs.'arest io'wn each covering the whole V.length ofthe'tuba but they may be arranged so that each/extends over one-half of the tube. The yibration 'of' the tube I6 is excited by magetostrictlve ifofrces due to the currents in the windingsiand iniord'er that these forces may have --vlthesaine fr eqliency as the exciting currents it .is necessary `that-"the tube be polarized magneticallybya undirectional magnetic iield. For this I purpose. apolarizing magnet 2| ris provided, the

vpole faces of' which are brought close to the ends offthev'ibratory kelement forming therewith a substantially closedl magnetic circuit. Small airgaps inustbelzeft between the ends of tube I 6 and ftrolled vby rheostat 24, produces the polarizing magnetic eld. To prevent eddy currents flowing lunder the action of the-exciting currents it is desirable to have the tube I6 slotted longitudinally as indicated at 20 in the ilgure.

The electrical impedance of a magnetostrictive element of the type illustrated, measured at the terminals of one of the exciting windings corresponds to that of the network illustrated in Fig. 5 comprising an inductance In shunted by a series resonant. impedance L1C1. The inductance Lo is that of the exciting Winding in the absence of any magnetostrictive action or with the tube I6 fixed everywhere so that it cannot vibrate. The vinductance L1 and the capacity C1 have values dependent onsthe physical dimensions and the mechanicalconstants of the tube I 6 and on the magnetostrictive force factor. Expressions for their values in particular cases are given by -Butterworth inthe Proceedings the Physical Society, March 1, 1931 page 166. It may be noted that the resonance of L1 and C1 corresponds to the first` mechanical resonance frequency of the where l, w, and t are respectively the length,

vided by the magnetostriction effect the frequenciesfo and fr are very close together thus tending to limit the use of the device to filters of very narrow band width. Increased separation of the critical frequencies can be obtained by the addition of a capacity in series with the exciting winding, the resulting reactance characteristic being of the type shown by curve 26 of Fig. 6. A new resonance frequency fz below fn is introduced and the higher resonance is moved upwards from f1 to fa with respect to the antiresonance frequency fo may be controlled by varying the value of the series capacity.

Fig. '7 illustrates the application of magnetostrictive vibrators of the type described above in a broad band filter in accordance with the invention. Two vibratory elements 21 and 28 are used, element 21 having two equal exciting windings Wi andA We which are connected respectively in the line branches of the lattice and element 28 having equal winding Wb and We connected in the lattice branches. Equal capacities C. and C.' are connected in series with W. and W.' respectively, and a corresponding pair of equal capacities Cb and Ch are included in the lattice branches. The vibratory members are shown in schematic form only, the polarizing magnets and their associated circuits being omitted.

Windings W. and We' are so poled that the currents flowing therein will normally aid each other in exciting mechanical vibrations. These windings are included in series in the circuit from filter terminal i through capacity C., winding W. to terminal 3 and thence through the filter load to terminal I, winding Wr', capacity C.' and ter- 25 minal 2. The poling of windings W. and W.'

must be such that a current traversing this circuit will magnetize the core in the same direction. I'he windings Wb and Wb' are poled in a similar manner. By virtue of the equality of the 40 two windings on each of the vibrators equal impedances are introduced into the two line branches of the network and likewise into the two lattice branches.

To provide a single transmission band it is necessary that the lattice branch. impedances have frequency characteristics different from those of the line branch impedances, but related thereto in such manner as to produce coincidences of the critical frequencies. As shown in U. S. Patent 1,828,454 of October 20, 1931, to H. W. Bode in connection with symmetrical lattice lters, the 'anti-resonance frequencies of the line branches must coincide with resonance frequencies of the lattice branches and vice versa, within the frequency range of the transmission band, whereas in the attenuation ranges the coincidences must be between critical frequencies of the same character. Two examples of possible arrangements of the critical frequencies in filters of the type shown in Fig. '1 are illustrated by Figs. 8 and 9. VIn Fig. 8 the full line curve 21 corresponds to the reactance of the line impedances and dotted curve 28 to the reactance of the lattice branches. In this case the transmission band extends from the 65 lowest critical frequency f. to the highest fb. In

Fig. 9 curves 29 and 30, which correspond respectively to the line and lattice branch impedances. have a direct coincidence of two resonances at frequencyje and an inverse coincidence of a resonance and an anti-resonance of fg. The band in this case extends from the lowest critical frequency le to the third critical frequency f4, the frequency f. being outside the band. In both figures the location of the'band is indicated by a shaded horizontal area.

What is claimed is:

1. A broad band wave filter comprising two pairs of equal impedance branches connected between two input terminals and two output terminals to form a symmetrical lattice network, a mechanical vibratory element symmetrically cou' pled electromechanically to both branches of one equal pair and a second mechanical vibratory element similarly coupled to the other pair of equal branches, said vibratory elements having different resonance frequencies.

2. A broad band wave filter in accordance with claim 1 in which the mechanical vibratory elements are comprised of piezoelectric crystal plates.

3. A broad band wave filter in accordance with claim 1 in which the mechanical vibratory eiements comprise piezoelectric crystals, each crystal being disposed symmetrically between two pairs of electrical plates to effect the electromechanical coupling to the pairs of equal impedance branches.

4. A broad band wave filter in accordance with claim 1 in which the mechanical vibratory elements are coupled electromagnetically to the respective pairs of branches.

5. A broad band wave filter in accordance with claim 1 in which the mechanically vibratory members comprise tubes ofi metal having magnetostrictive properties and in which the coupling to the respective impedance branches is effected by inductive windings extending over the length of the tubes.

6. A broad band electric wave filter comprising va plurality of connected impedance branches,

two of said branches being of equal impedance and being subject to oscillations of like phase and amplitude when waves are transmitted through the filter, and a tuned mechanical vibratory element coupled electromechanicaliy and in equal degrees to said two impedance branches whereby it modifies the impedances of said two branches equally;

7. In a broad band electric wave filter comprising a plurality of impedance branches, said branches having frequency dependent impedances the characteristics of which deiine a transmission band between definite frequency limits, two impedance branches having equal impedances and being subject to oscillations of like phase and amplitude when waves are transmitted through the filter. and a tuned mechanical vibratory element coupled electromechanically and in equal degrees to said two impedance branches whereby it modifies .the impedances thereof equally.

WARREN P. MASON. 

